Our EcoCheck series takes the pulse of some of Australia’s most important ecosystems to find out if they’re in good health or on the wane.
Queensland’s Brigalow Belt is among Australia’s most significant biodiversity hotspots. Extending over an area of 36.4 million hectares from Townsville down into New South Wales, it was famously where the explorer Ludwig Leichhardt travelled, prickly pear was vanquished, and the now-extinct paradise parrot once lived.
Although the region contains diverse ecosystems, from dry vine scrub to grasslands, it is named after the species of tree that once dominated: the brigalow (Acacia harpophylla). This unusual, long-lived acacia with its dark, fissured bark and distinctive silver leaves forms dense woodlands, home to unique and threatened plants and animals.
Before clearing, brigalow-dominated ecological communities covered an estimated 7.5 million ha within the Brigalow bioregion. But those vast brigalow woodlands are no longer here.
Since the arrival of Europeans in the 1850s, 90% of brigalow forest has been cleared. Brigalow grows on fertile, cracking clay soils – the same soils needed for agriculture. Only 790,000 ha of brigalow ecosystems remain – just over 10% of the original extent. Sixteen out of 22 ecosystems where brigalow is the dominant or co-dominant species have less than 10% left – and even those are under threat.
Clearing of brigalow for crops and pasture began soon after European settlement. Initially, the task of turning the Brigalow into a breadbasket turned out to be more challenging than the settlers expected. Brigalow trees have a well-developed lateral root system. If the tree or roots are damaged, dense “suckers” spring up. This growth stage can last for 20-30 years and is followed by a “whipstick” stage lasting another 20-30 years before mature forest is formed.
This habit made permanent removal very difficult, as suckers can occur at a density of 20,000 stems per hectare.
Agricultural development was also delayed by the invasion by prickly pear. Between 1901 and 1925, these spiky American cacti spread across 24 million ha of Queensland and NSW. Communities and governments despaired of being able to control this weed, but by 1932 a biological control agent, the Cactoblastis moth, had almost completely destroyed prickly pear.
It was not until the 1960s – and a “perfect storm” of mechanised land clearing, favourable government policies, scientific research into brigalow control, and a push for agricultural development – that clearing could occur on a grand scale. Once the problem was cracked, clearing rates soared. At times, rates equalled those in tropical forest regions such as the Amazon and Southeast Asia.
Legacy of loss
Today, the Brigalow Belt is a precious, but threatened, reservoir of endemic diversity. Brigalow woodland is nationally endangered, with severe consequences for the animals of the Brigalow Belt. Four species, including the paradise parrot, are extinct. Another 17 are on the threatened species list in either NSW or Queensland.
Remaining patches of brigalow are often modified by the removal of understorey shrubs and fallen timber. This affects habitat structure for reptiles and woodland birds in particular, reducing population sizes and encouraging aggressive competitors such as the noisy miner.
Many exotic species have been introduced, including pasture grasses. The most widespread of these is buffel grass, which has been a boon for pastoralists. Unfortunately, its invasion of remnant brigalow and contribution to fuelling bushfires has had dramatic effects on plant and animal biodiversity.
The Brigalow Belt is also home to 13 reptile species that are found only in this region, and another 14 for which the region is their main home. Eleven of the 148 reptile species found in the Brigalow Belt are threatened.
But the very suckering habit that made brigalow trees so difficult to clear in the early days may now be its salvation. Although brigalow regrowth is initially very different from old-growth woodland, if it is allowed to persist, the vegetation structure becomes more and more complex and diverse. After 30-50 years, mature regrowth can support as many bird species as old-growth woodland.
The future of the brigalow
Only 1% of the remaining brigalow woodland is in protected areas. The rest is highly fragmented, existing mainly as tiny patches, linear strips along roads and fence lines, and areas of regrowth.
Land-use change for agriculture, coal mining and coal seam gas extraction continues to nibble away at remaining brigalow ecosystems, despite protection by state and federal laws. In 2013-14, 44% of all woody vegetation clearing in Queensland occurred in the Brigalow Belt.
Legislation controlling most broadscale clearing of remnant native vegetation was introduced through the Queensland Vegetation Management Act 1999. This phased out clearing of remnant vegetation by December 2006. In 2008, recognising that the only way to recover threatened ecosystems like brigalow forest was to increase their extent, mature “high-value” regrowth of threatened ecosystems was also protected.
But in 2013 came a setback, with the introduction of the Vegetation Management Framework Amendment Act 2013, which allowed for much more vegetation clearing and removed the protections for high-value regrowth. Laws to reinstate those protections are before the Queensland Parliament.
The opportunity to recover the brigalow will not last forever. With repeated clearing, burning and cultivation, these forests could eventually disappear for good. But in those areas where some resilient regrowth remains, there is potential for recovery.
In 2009, there was an estimated 7,226 square km of regrowth, comprising a range of structures from juvenile bushes (aged 5-10 years) to almost mature stands (aged 30-50 years). This regrowth provides a promising and cost-effective way to increase habitat area for both fauna and flora, and reduce their risk of extinction.
To do this, however, we need to find ways to make retention of brigalow regrowth attractive and valuable to landholders, through stewardship schemes or carbon offsets. Only then might the Brigalow Belt bounce back.
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Leonie Seabrook, Landscape Ecologist, The University of Queensland; Clive McAlpine, Senior Research Fellow in Ecology, The University of Queensland, and Martine Maron, ARC Future Fellow and Associate Professor of Environmental Management, The University of Queensland